Method for the production of high strength notch tough steel

ABSTRACT

A fully killed steel having a high yield strength and high notch toughness below 0*F. and containing by weight up to 0.20 percent carbon, at least 0.015 percent columbium and at least 0.75 percent manganese, said steel having a substantially uniform, fine grain, essentially ferritic grain structure with an average grain size of finer than 9.0 as determined by ASTM Test No. E 112, said steel preferably having a longitudinal Charpy Impact of at least 60, as well as a method of producing said steel which essentially involves continuously rolling the steel in the temperature interval 1700* to 1900*F.

United States Patent Shaughnessy et a1.

[ 1 July 29, 1975 METHOD FOR THE PRODUCTION OF HIGH STRENGTH NOTCH TOUGH STEEL Inventors: Reginald N. Shaughnessy; Robert W. Witty; Robert J. Ackert, all of Sault Sainte Marie, Canada Assignee: The Algoma Steel Corporation,

Limited, Canada Filed: May 11, 1973 App]. No.: 359,423

Related U.S. Application Data Continuation-impart of Ser No. 253,897, May 16, 1972, abandoned.

Foreign Application Priority Data May 12, 1972 Canada 147942 References Cited UNITED STATES PATENTS 5/1939 Becket 75/126 F 3,010,822 11/1961 Altenburger et a1. 75/126 F 3,102,831 9/1963 Tisdale 75/123 N 3,386,862 6/1968 Johnston et a1.... 148/36 X 3,539,404 11/1970 Retana 148/12.4 3,619,303 11/1971 Semel 75/126 F 3,681,057 8/1972 Kawakami 75/124 3,721,587 3/1973 Allten 148/36 3,738,874 6/1973 Allten 148/12 F 3,834,949 9/1974 Heitmann et al 148/12 F Primary Examiner-L. Dewayne Rutledge Assistant ExaminerArthur J. Steiner Attorney, Agent, or Firm-Spencer & Kaye 5 7 ABSTRACT A fully killed steel having a high yield strength and high notch toughness below 0F. and containing by weight up to 0.20 percent carbon, at least 0.015 percent columbium and at least 0.75 percent manganese, said steel having a substantially uniform, fine grain, essentially ferritic grain structure with an average grain size of finer than 9.0 as determined by ASTM Test No. E l 12, said steel preferably having a longitudinal Charpy Impact of at least 60, as well as a method of producing said steel which essentially involves continuously rolling the steel in the temperature interval 1700 to 1900F.

6 Claims, 1 Drawing Figure PATENTEU JUL28 1975 E Q E528 235238 a 6 a a OOI METHOD FOR THE PRODUCTION OF HIGH STRENGTH NOTCH TOUGH STEEL CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of abandoned application Ser. No. 253,897, filed May 16th, 1972.

The present invention relates to steel. In particular the present invention relates to steel of improved yield strength and notched toughness at low temperatures, particularly below F. and a method of making said steel.

There is an ever present and increasing requirement for steels of improved strength and notch toughness and with the development of the arctic regions, such as arctic gas and oil field reserves, steels with high notch toughness at low temperatures and yield strength have become increasingly desirable. For example in the production of pipelines.

Heretofore it has been conventional in producing such steels by subjecting mild steels modified with a suitable grain refiner such as columbium and/or vanadium at levels totalling approximately 0.05 percent by weight of the steel to a controlled rolling process. Conventionally controlled rolling processes involve heating the ingot or slab to a temperature in excess of 2250F. and rolling the ingot or slab with a rolling schedule involving delays in the pass sequence such that substantial reduction in thickness occurs below a temperature in the range l700 to 1750F. finishing with a temperature of approximately 1450C. Typically these delays are introduced in the temperature range 1700F. to 1900F. and are necessary to assure sufficient reduction at the lower temperature to produce a fine grain structure. The degree of reduction required below 1550F. introduces difficulties in producing flat plate. In addition the properties achieved'in the steel are less consistent than desirable due to the necessity of precise temperature and process control. For this reason it is often necessary to normalize the plates produced in this manner especially when the thickness of the rolled plates is /2 inch or greater. This normalizing process is of considerable additional expense both cost-wise and time-wise in the process. Still further, low temperature, notch toughness and strength of low alloy, mild steel plate produced by conventional controlled rolling with or without normalizing is relatively low and substantial improvement is desirable.

The present invention provides an improved steel of substantially higher notch toughness at low temperatures, i.e. below 0F. which make it highly suitable for structures which are subject to low temperatures, eg in the arctic such as pipelines and power transmission towers, which steels may be produced by a simple and more efficient rolling process than the conventional controlled rolling process.

It has now been found that an improved steel plate can be made by rolling an ingot or slab of steel having a low carbon and high columbium content according to a conventional rolling technique provided that the steel is rolled continuously in the temperature interval 1700l900F. and preferably 1700-1950F.

According to the present invention therefore, there is provided a method of making a high strength steel of improved notch toughness at temperatures below 0F. which method comprises subjecting an ingot or slab of said steel of chemical composition set forth hereinafter and at a temperature of at least 2100F., preferably at least 2250F., to a reduced rolling, said rolling being continuously effected through the temperature range 1900F. to l700F.

The present invention also provides a fully killed steel having a high yield strength a high notch toughness below 0F. and containing up to 0.20 percent by weight carbon at least 0.75 percent manganese and at least 0.015 percent by weight columbium, said steel having an essentially uniformly fine grain essentially ferritic grain structure with an average grain size of finer than 9.0 as determined by ASTM Test No. E 1 12.

The present invention provides, according to a particular embodiment of the present invention, a fullykilled steel having a high yield strength and high notch toughness below 01 and consisting essentially of by weight 0.02 to 0.12 percent carbon, 0.09 to 0.18 percent columbium, 1.0 to 2.0 percent manganese, 0 to 5 percent molybdenum, not more than 0.025 percent phosphorus, not more than 0.035 percent sulphur, 0 to 0.6 percent copper, 0 to 1.0 percent nickel, 0 to 1.0 percent chromium, 0 to 0.2 percent zirconium, 0 to 0.06 percent rare earth metal, the balance being iron and incidental impurities said steel having a substantially uniform fine grain substantially ferritic grain structure with an average grain size finer than 9 as determined by ASTM Test No. E l 12.

The carbon is present in the steel in an amount up to about 0.20 percent and preferably from about 0.02 percent by weight to 0.12 percent by weight. In amounts above 0.12 percent by weight the steel tends to lose its high notch toughness due to the formation of substantial amounts of pearlite grains in the structure of the steel and further it is found that the impact strength of the steel is also substantially reduced as is its weldability which is highly desirable in the steel plate. It is also found that the low carbon content raises the A transformation temperature and thus reduces the crystallization of austenite subsequent to the final rolling of the steel. The lower the carbon content of the steel, the better the notch toughness. The manganese content of the steel however is usually provided by means of a ferro manganese addition containing carbon as an impurity and there is a practical lower limit for the carbon present in the steel. Preferably the carbon is present in an amount from 0.04 to 0.10 percent and more preferably 0.06 to 0.07 percent by weight. It has been found that the fine grain structure in the steel which is predicated primarily on the columbium content is still achieved with steels having a carbon content up to about .20 percent despite the formation of pearlite grains in the structure and the steel still hasacceptably high impact strength and percentage shear, particularly when the carbon content is below 0.16 percent. However, to maintain weldability with such high carbon content in the steel, the columbium content must be reduced and thus a high carbon content in the steel which lowers impact strength and percentage shear in the steel and a low columbium content which also lowers the impact strength and percentage shear in the steel is not preferred.

The columbium is preferably present in the steel in an amount from 0.09 to 0.18 percent and more preferably in the range 0.12 to 0.15 percent by weight of the steel. It is believed the net result of the low carbon content and the higher than conventional columbium is to provide a volume fraction of columbium carbide and columbium carbo-nitride precipitates in the steel sufficient to retard austenite grain growth at high tempera tures. It is believed that the combination of low carbon content and high columbium content in the steel affords a greater potential for the precipitation of columbium carbo-nitride and columbium carbide in the temperature range l700F. to l900F. because the solubility limit of columbium in austenitic steel increases with decrease of carbon content, i.e. a greater volume fraction of the precipitate of columbium carbide and columbium carbo-nitride is present at a given temperature when the carbon content of the steel is decreased and the columbium content increased. It is thus found that by continuo usly reducing the steel in the temperature interval l700F. to l900F. by a rolling schedule such as to produce 50 percent reduction in the thickness of the steel below about 1800F. a very fine essentially uniform grain size is achieved and maintained. With a columbium content of less than 0.09 percent by weight the degree of grain refinement which is capable of being achieved in the austenite is reduced. It is possible however to achieve the fine grain structure steel by the rolling technique of the present invention with substantially less columbium than 0.09 percent with consequent high impact strength and high percentage shear that is required. In particular, it has been found that the columbium content may be as low as 0.015 percent by weight and still achieve the fine grain structure with high percentage shear and high impact strength. Suitably, the columbium content is at least 0.04 percent and more suitably at least 0.06 percent by weight. It is found that the impact strength and percentage shear increase with percentage columbium and above about 0.06 percent it is believed the colubmium acts inter alia to increase ultimate tensile strength.

The manganese is present in the steel to provide strength including both yield and ultimate tensile strength and in particular the manganese is believed to provide strength in the ferrite grains in the steel. The strength of the steel is determined to some extent by the thickness of the steel plate and the amount of manganese present in the steel. In general the thicker the steel the lower its yield strength and ultimate tensile strength for a particular chemical composition. By raising the manganese content of the steel, the strength of the steel is thus increased. In general it has been found for thin steel plate of less than 0.42 inches to obtain a high strength steel with a yield strength in excess of about 60,000 psi the manganese content should preferably be in excess of about 1 percent and for thicker steel should be at least 1.0 to 2.0 percent by weight. However, a manganese content of the steel may be as low as about 0.75 percent and still achieve the advantages of the present invention. A manganese content below 0.75 percent yields a steel which tends to have a low percentage shear. The maximum manganese contents is determined to a great extent by the carbon equivalent required in the steel for weldability. Preferably the manganese content of the steel for thin steel plate is in the range 1.30 to 1.70 percent by weight and more preferably 1.30 to 1.45 percent by weight and for thicker steel in excess of 0.42 inches the manganese content is in the range 1.60 to 2.0 percent by weight. Where it is desired to increase the ultimate tensile strength molybdenum will also be present in the steel, suitably in an amount not in excess of about 0.5 percent by weight. The molybdenum is found to increase the ultimate tensile strength but to slightly decrease the yield strength of the as-rolled steel. The presence of molybdenum is particularly useful in thicker plates to increase the ultimate tensile strength.

Sulfur is present as an impurity in steels and lowers the transverse impact strength of the steel and, in particular, combines with the manganese present in the steel to form manganese sulphide stringers which extend longitudinally in the steel causing directionability of impact properties. The transverse impact strength is increased by adding rare earth metals which are capable of reacting with sulfur in the molten steel. Rare earth metals also react with oxygen in the steel, and the steel is killed suitably by the addition of killing agents such as aluminum or by the addition of larger amounts of the rare earth than are necessary to react with the sulfur. The rare earth thus can be used both as a killing agent and for combination with the sulfur. Desirably the rare earth metals are present in amounts up to 0.06 percent by weight of the steel. Zirconium at levels up to 0.20 percent and preferably up to 0.10 percent may be used in place of rare earth metals as a sulphide modifier and/or a killing agent. Aluminum is the most useful killing agent and is present in an amount sufficient to kill the steel. When too little aluminum is added, the steel is not killed and when too much aluminum is present surface problems may occur in the killed steel. The aluminum is suitably present in the steel as killing agent in an amount from 0.02 to 0.10 percent by weight and preferably from 0.03 to 0.06 percent by weight. The sulfur content of the steel is maintained below 0.035 percent by weight and desirably below 0.020 percent by weight.

Phosphorus is also an impurity in the steel and tends to render the steel brittle and the content of the phosphorus in the steel is maintained below 0.025 percent by weight and desirably below about 0.015 percent by weight.

The steel may also contain conventional strengthening ingredients such as copper, nickel and chromium or mixtures thereof in levels up to 1.0 percent each. Preferably the copper is present in an amount from 0.2 percent by weight to 0.6 percent by weight and preferably up to 0.4 percent by weight. A further such agent which may be mentioned is vanadium which may be present in an amount up to about 0.12 percent by weight.

The rolling of the steel for ingot or slab to plate may be effected on roughing mills and finishing mills with the transfer between the mills being effected at temperatures outside the interval l900F. to l700F. Alternatively the rolling can be effected on a single mill. It is a critical feature of the present invention that the steel be continuously rolled in the temperature interval from about l900F. to about l700F. and preferably about l950F. to about l700F. It has been found still further preferable to roll continuously until the steel temperature has reached l650F. It is further stated in the parent application Ser. No. 253,897 that in the continuous rolling that there should be no undue delay and that the transfer between the roughing mills and the finishing mills should be within the aforesaid temperature interval. It should be explained however that provided the transfer between the mills is rapid, e.g. of the order of less than about 20 seconds, the transfer may take place at temperatures down to about l850 F. without detracting significantly from the fine grain'structure and impact and percentage shear of the steel produced.

in the rolling of the steel the initial temperature of the ingot may be at least 2 lOF. and preferably at least that this uniform fine grain size is present in thin rolled plate steel of thickness of about Vs inch and in thicker rolled plate steel in the range V2 inch to about 1 /2 inches. It is believed that it is this uniform fine grain fer- 2250F. in order to ensure that the columbium is main- 5 ritic structure in the steel which contributes particutained-in solution in the austenitic steel, and allows the larly to its high notch strength particularly at low temcolumbium to precipitate out as columbium carbide peratures in the range 0F. to 50F. Thus it will be and/or columbium carbonitride particles at a temperaseen from the following Examples that at temperatures ture in the range l700 to l900F. when continuous below 0F. that the steels have longitudinal Charpy imrolling of the steel causes the columbium carbide andpactValues of at least 60 ft. lbs., preferably at least 80 /or columbium carbonitride to precipitate and thus reft. lbs., more preferably at least 190 ft. lbs. and even fine austenitic grain structure. Thus it is a critical feamore preferably at least 200 ft. lbs. with a percentage ture of the present invention that the slab be continushear of at least 90 percent, and transverse Charpy lmously rolled in the temperature range 1900F. to pact Values of at least 60 ft. lbs. at OF., at least 50 ft. l700F. and preferably l950 to l700F. with no undue lbs. at F. and at least40 at -50F. delay between passes. Desirably there will be at least 50 The Present invention will be further illustrated y percent reduction of the thickness of the steel plate at I y of the following Examplesa temperature below 1800F. It is believed that a conventional steel rolled according to the known con- EXAMPLE 1 trolled rolling process cannot produce and maintain a 20 Three P a e e e prep re fro n electric are furflne austenitic grain size above l700F. The rolling of naee heat of the ollowing composition: the conventional steel from l700F. down to about Element Amount l450F. serves to develop a fine grain structure in the rolled steel plate. In contrast thereto with the rolling Carbon 0.07% reduction process of the present invention in combinagffigfifii' tion with the low carbon content of the steel the forma- Phosphorus 01010 2 tion of pearlite during cooling of the steel is substang g m 8-933: tially reduced and the high columbium content of the Muminum 1 mm steel allows for fine grain particles of columbium carand Impurities Balance bide and/or columbium carbonitride (the nitrogen being present as a natural impurity in the steel). By Three slabs 54 inches X 7 inches X 86 inches'were continuously rolling the steel in the range 1700 to rolled in a 2 HI roughing mill and then in a 4 HI finishl900F. and preferably 1700 to 1950F. this ensures ing mill a final rolled) Plate Size 102 inCheS X /8 that the columbium carbide and/or columbium carinch X 447 inches according to the schedule set forth bonitride particles precipitate out and provide a subin the following Table 1. stantially uniformly fine essentially ferritic grain steel Of these plates, plate 544 which was continuously having improved notch toughness at low temperatures rolled in the temperature range l500-2040F. has the below 0F. and also below -50F. best combination of properties, maintaining 83 ft. lbs.

The finishing temperature of the steel in the process impact energy with 95 percent shear in the fracture surof the present invention is usually substantially higher, face down to -50F. e.g. in the range 1500 to l600F. thus making the pro- Plate 545 was essentially continuously rolled from duction of flat steel plate simpler than in the conven- 2200F. down to 1760F. Although good impact proptional controlled rolling process. However it is within erties were measured at 0F., the percent shear area is the scope of the present invention to continue the rolllower at all temperatures as compared to plate 544. ing to lower temperatures as in the controlled rolling Plate 541 was not rolled in the temperature range process to improve the toughness and strength properl8571942F. and displayed impact properties infeties of the steel plate even further. rior to those of plates 544 and 545.

Table l 2 HI Mill 4 HI Mill Tensile Results Plate No. Entry Temp. Exit Temp. Entry Temp. Exit Temp. Yield (psi) U.T.S. (psi) ElongationU/r in 2") Charpy impact Values Longitudinal Transverse 0F. 20Fv F OF. 2(lF. Plate No. ft. lb. 7: shear ft. lb. "/1 shear ft. lb. 74 shear ft. lb. ft. lb. ft. lb.

It has been found that the steel produced by the pres- EXAMPLE 2 ent invention has a very fine grain structure which is substantially uniform with an average grain size of finer than 9 and preferably at least 9.5 and more, at least 10 as determined by ASTM Test N0. E112 and it is found Five plates of approximately V2 inch thickness were prepared from a basic oxygen steel plant heat of the following composition:

7 8 Element Amount The steel slabs were first rolled in a 2 HI roughing Carbon 010% mill and then on a4 Hlfinishmg mill to a final (as Manganese 113% rolled) plate size of 139 mches X 0.42 mches X 770 Silicon 0.04% I Sulphur 002092 5 inches according to the schedule set forth in Table 3. Phosphorus 0.007% Columblum 0.11% The tensile strength and the Charpy Impact Values of Aluminum 0.050% on and Impurities Balance the steel plate obtained are shown in Table 3. The substantially improved Charpy Impact Values of the steel Five slabs of the aforesaid composition were rolled Plates are those of EXamPle 3 are believed to be due to on a 2H1 roughing mill and then on a 4 HI finishing mill. the Preseme of the rare earth metals -gcerium) The rolling temperature schedule used and the pro which reduces the sulphur content of the steel and reties obtained are summarized in the following Table 2. duces the effect of the Sulphur remaining the Steel- It will be seen from Table 2 that the plates which were all continuously rolled in the temperature interval The Steel had a Substantially uniform ferritic grain 1900F to 1700F had Charpy Impact values in ex ss structure of very fine grain size of 12 as determined by of 80 ft. lbs. and a percent shear of at least 90 percent. ASTM E 1 Table 2 2H1 4H1 Longitudinal Entry Exit Entry Exit Yield U.T.S. Elong. Impact Results at F. Plate No. Temp. Temp. Temp. Temp. Gauge ft.(psi) (psi) in 2") ft. lb. shear Table 3 2 HI MILL 4 HI MILL TENSILE RESULTS Plate Entry Exit Entry Exit Yield (psi) UTS (psi) Element'iv 2"% No. Temp. Temp Temp. Gauge Temp Long Trans Long Trans Long Trans Charpy Impact Values Longitudinal Transverse Plate 0F. 20F. -F. 0F. 2 F. 50F. N0. Ft.Lb. %Shear Ft.Lb. %Shear Ft.Lb. %Shear Ft.Lb. %Shear Ft.Lb. %Shear Ft.Lb. %Shear EXAMPLE 3 45 EXAMPLE 4' Five steel plates were prepared from a basic oxygen steel plant heat with a slab size 74 inches X 7 inches X 102 inches of a steel of the following composition:

E] t Amount men 55 bly higher than plate 31068. Carbon 0.07% Plate 39087 represents a plate rolled with no controlgg ggfi 323: ling restrictions placed on the 4 HI mill. Sulphur 0.010% Table 4 shows the definite improvement in the imphosphql'us Q0099? pact properties achieved in plate 39716. Columbrum 0.13% v Aluminum 0.06% 60 It was observed that plate 39716 had the best microf l g- 3 structure, being completely free of the undesirable 21 cc an mpun 3 n bands of coarse grain observed in the other plates.

Table 4 2 HI 4 H1 P]. No. Entry Exit Entry Exit Gauge Yield U.T.S. Elong. Long. Impact at -20F. Temp. Temp. Temp. Temp. (in.) (psi) (psi) ft. lb. "/1 shear EXAMPLE 18 inches X 7 /2 inches X 40 lb/ft. wide flange structurals were produced from a basic oxygen steel plant heat of the following compositions:

Element Amount Carbon 0.07%

Manganese 1.39% 1 Silicon 0.10% 0 Sulphur 0.018%

Phosphorus 0.008%

Columbium 0.13%

Aluminum 0.050%

Cerium Nil lron and impurities Balance A 22 inches X 15 inches bloom was rolled into 22 /2 inches X 11 inches X 4 inches blanks which were reheated to 2100F., broken down in a breakdown mill and then transferred to a parallel flange mill where it was rolled continuously through the temperature range 1600l950F.

While the present invention has been described with reference to plate production it also has application to the production of sheet products in general and rolled shapes such as structural products, e.g. l-beams, wide flange beams and merchant shapes.

The temperatures referred to herein throughout the disclosure and claims refer to the mean temperature of the steel being rolled and not the surface temperature of the steel which is typically 50F to 75F lower than the mean temperature. In the Examples herein, the

mean temperature of the steel was calculated by computer based on the mean reheat soaking temperature which can be fairly accurately determined. The computer makes adjustments to the calculated mean temperature of the steel at any particular place in the rolling procedure based upon hardness mill operating parameter such as rolling speeds, measured wall forces (e.g. hardness of the steel forming the plate) water sprays, etc. Such computer control of the rolling procedure and the calculation of the mean temperature of the steel from the rolling history and properties of the steel plate are known techniques inter alia to Canadian General Electric Company and do not form part of the present invention.

Operating the process of the present invention using pyrometers which read the surface temperature of the steel only 'it is possible to calculate the mean temperature of the steel based on the prior history of the rolling procedure of the steel usually to within an error of +-20F with the pyrometers properly set up and calibrated. Thus, in rolling mills operating with pyrometers, i.e. without computer control may operate in accordance with the present invention by makingappropriate adjustments to the critical temperature range as measured by the pyrometers such as for example continuously rolling in the temperature range about 1850F to about 1650F based on the assumption that there is 50F temperature difference between the surface temperature of the steel and the mean temperature of the steel.

EXAMPLE 6 Four plates of the thickness setforth in the following Table 6A were prepared from four basic oxygen steel plant heats of the composition set forth in the following Table 6.

The steel in each of the heats was rare earth treated so as to contain sufficient rare earth metal values to achieve to percent sulphide modification.

A slab of eachof the aforesaid heats was rolled on a 2H1 roughing mill and then a 41-11 finishing mill. The slab entering the 2H1 roughing mill was 8.9 inches thick, entered the mill at a temperature of 2234F and was rolled to a thickness of 3.4 inches exiting from the mill at a temperature of 1934F. The 3.4 inch -.thick plate was then immediately passed to the 41-11 finishing mill entering ata temperature of 1876F and was rolled to the final thickness shown in Table 6A exiting at a temperature of 1645F.

The results obtained are shown in Table 6A.

Table 6A CHARPY IMPACT RESULTS TEST PLATE HEAT PLATE YIELD U.T.S. ELONG LONGITUDINAL TRANSVERSE TEMP. THICK NO. NO. (PS1) (PS1) (8") FT. LBS. SHEAR FT. LBS. %SHEAR (F) (1N) F1237 19139 68570 84410 21 115176153 100100100 62 51 54 .50 40 40 20 F1403 19998 68340 86060 25 116 123 65 65 65 54 74 68 95 95 95 -20 D7085 14497 62800 79400 26 113 106 100 100 100 78 93 103 100 100 100 0 X1 F1057 14498 61800 77570 25 195 137 100 100 100 100 100 100 100 100 100 0 W4 EXAMPLE 7 In a similar manner as in Example 6 plates 0.500 to 0.75 inches thick were rolled from a basic oxygen steel Each heat was rare earth treated in a similar manner to Example 6. Each heat was continuously rolled for reduction from 1910F to a finishing temperature of approximately 1550F with a greater than 50 percent reduction below 1800F.

The plates so obtained had the following properties.

On rolling a slab of similar composition except for the absence of columbium according to the same schedule the plate obtained had the following properties.

Impacts 31 ft. lb. and 25% shear -20F Grain Size (/2 Plate) Surface -ASTM 8.0 Core ASTM 8.0.

It will be seen that the presence of columbium is essential for the production of the steel of the present invention of fine grain size.

The Figure represents a graph for V2 inch plate of columbium content of the steel against percentage shear in a standard Charpy sample broken at 20F, the results being taken from the preceeding Examples. It will be seen that the rolling process of the present invention is effective to some degree on steel at columbium levels from to 0.14 percent. For optimum results the steel of /2 inch thickness should contain at least 0.04 percent columbium. This optimum columbium level varies with plate thickness being higher for thinner plates and vice versa.

Physical Properties (Heat Average) Yield Strength 66,500 psi Ultimate Tensile Strength 78,400 psi Elongation 32% in 2 inches Impact Properties (For full size Charpy specimens) Longitudinal Ft.lbs. 219 207 177 170 150 91 Shear 100 100 95 80 65 23 Transverse Ft.lbs. *N.T. l 18 1 16 94 I00 *N.T.

Shear *N.T. 73 72 42 49 *N.T. Microstructure: Grain size (.75" Plate) Surface ASTM No. 105

Core ASTM No.

'N.T. not tested EXAMPLE 8 The embodiments of the invention in which an exclu- In a similar manner to Example 7 plates 0.500 inches thick were rolled by precisely the same schedule as in Example 7 from slabs of a heat of the following composition:

The heat was not rare earth treated. The plate obtained had the following properties.

Physical Properties Yield Strength 64,400

Ultimate Tensile Strength 80,150

Elongation in 8" Impact Properties (For full size Charpy specimens) 0 O O 20 F 50 F F Longitudinal Ft.lbs. 82 62 29 30 Shear 95 30 5 Microstructure Grain Size (/2" Plate) Surface ASTM No. 12.5 Core ASTM No. 12.0

0 sive property or privilege is claimed are defined as follows:

1. A method of making a high strength steel of improved notch toughness below 0F which comprises subjecting a fully killed steel consisting essentially of the following composition by weight:

carbon 0.02 to 0.20% manganese 0.75 to 2.0% columbium 0.015 to 0.18% aluminum 0.02 to 0.10% nickel O to .0 chromium 0 to 1.0 copper 0 to 1.0 molybdenum 0 to 0.5 vanadium 0 to 0.12% zirconium 0 to .20% rare earth metals 0 to 06% balance iron and impurities to heating to at least 2100F, and subsequently rolling said steel to continuously reduce the steel, without any substantial time delays, throughout the temperature range of from l900 to l700F with at least a 50 percent reduction in section at below 1800F, said steel having a uniform fine grain of an essentially ferritic grain structure with an average grain size of finer than 9.0 as determined by ASTM No. E 112.

2. A method as claimed in claim 1 in which the rolling is continuous throughout the temperature range about 1950F to about 1700F of the steel.

3. A method as claimed in claim 1 in which the reducing rolling is effected in both a roughing mill and subsequently in a finishing mill, the transfer of the steel from the roughing mill to the finishing mill being above or below the temperature range l900F to l700F of the steel.

4. A method as claimed in claim 1 in which the initial temperature of the ingot is at least about 2250F.

ing is continuous throughout the temperature range about l9S0F to about l650F of the steel.

i l l l l 

1. A METHOD OF MAKING A HIGH STRENGTH STEEL OF IMPROVED NOTCH TOUGHNESS BELOW 0*F WHICH COMPRISES SUBJECTING A FULLY KILLED STEEL CONSISTING ESSENTIALLY OF THE FOLLOWING COMPOSITION BY WEIGHT:
 2. A method as claimed in claim 1 in which the rolling is continuous throughout the temperature range about 1950*F to about 1700*F of the steel.
 3. A method as claimed in claim 1 in which the reducing rolling is effected in both a roughing mill and subsequently in a finishing mill, the transfer of the steel from the roughing mill to the finishing mill being above or below the temperature range 1900*F to 1700*F of the steel.
 4. A method as claimed in claim 1 in which the initial temperature of the ingot is at least about 2250*F.
 5. A method as claimed in claim 1 in which the rolling is continuous throughout the temperature range about 1900*F to about 1650*F of the steel.
 6. A method as claimed in claim 1 in which the rolling is continuous throughout the temperature range about 1950*F to about 1650*F of the steel. 